Conventional BCM power supplies, such as boost, flyback, and buck converters, require a single pin to achieve Zero Current Detection (ZCD) for switching a power switch. FIG. 1 shows a conventional boost BCM power supply, in which a rectifier 10 rectifies an Alternating-Current (AC) voltage Vac to generate an input voltage Vin, an inductor L1, a power switch M, and a diode D1 establish an asynchronous power stage, a control Integrated Circuit (IC) 12 has a pin 14 for controlling the power switch M1 so that the asynchronous power stage converts the input voltage Vin into an output voltage Vo, an auxiliary winding La senses the change in the across voltage of the inductor L1 and asserts a zero-current detected signal VZCD when the current IL in the inductor L1 becomes zero to apply to a pin 16 of the control IC 12, and the control IC 12 identifies the timing of the inductor current IL that decreases to zero depending on the zero-current detected signal VZCD, and turns on the power switch M1 at the moment that the inductor current IL decreases to zero.
FIG. 2 shows a conventional quasi-resonant flyback BCM power supply, in which a rectifier 10 rectifies an AC voltage Vac to generate an input voltage Vin, a winding Lp at the primary side is connected between the rectifier 10 and a power switch M1, a winding Ls at the secondary side is connected between an output terminal Vo and a ground terminal, a control IC 20 has a pin 22 for switching the power switch M1 for converting the input voltage Vin into an output voltage Vo, an auxiliary winding La senses the current Is in the inductor Ls for asserting a zero-current detected signal VZCD to apply a pin 24 of the control IC 20 for the control IC 20 to identify the timing of the inductor current Is that decreases to zero, and the control IC 20 turns on the power switch M1 at the moment that the inductor current IL decreases to zero.
FIG. 3 shows a conventional buck BCM power supply, in which a control IC 30 has a pin 32 for switching a power switch M1 to charge and discharge an inductor L for generating a stable current ILED for a LED string 36, a filter 38 composed of resistors Rf1, Rf2, and Rf3 and a capacitor Cf detects the current IL in an inductor L1 and asserts a zero-current detected signal VZCD when the inductor current IL decreases to zero to apply to a pin 34 of the control IC 30, and the control IC 30 turns on the power switch M1 responsive to the zero-current detected signal VZCD.
As illustrated in FIGS. 1 to 3, a conventional BCM power supply needs two pins to drive the power switch M1 and detect the zero current, respectively. If a pin can both drive the power switch M1 and detect the zero current, the pin number of the control IC and the cost thereof can be reduced. U.S. Pat. No. 7,355,373 disclosed using a signal on a multiple function pin to both drive the power switch and detect the zero current. However, the disclosed control IC cannot turn on the power switch, and an external circuit of the control IC is required to turn on the power switch instead. Accordingly, users have to set parameters of the external circuit by themselves, so that the power switch can be turned on and turned off, and this brings inconvenience to the users.